Suddenly Specter’s path to reelection as a Democrat is looking far from certain, as progressives recoil at some aspects of Specter’s voting record and two prominent candidates refuse to bow out. On the GOP side, the more Republicans look at the Pennsylvania political landscape, the more questions they have about whether Toomey is the right fit against Specter in a general election.

Specter’s initial warm welcome from his new party has given way to a more tepid greeting, as liberal groups chafe at Specter’s continued opposition to key elements of the party agenda — like the $3.4 trillion budget, which he voted against last week, and the Employee Free Choice Act.

Specter did little to ingratiate himself with his new party colleagues over the weekend, when he disputed a Wall Street Journal report that said he promised President Barack Obama he would loyally support his agenda.

“I did not say I would be a loyal Democrat. I did not say that,” Specter insisted Sunday on “Meet the Press.”

Rep. Joe Sestak (D-Pa.), a well-funded Philadelphia-area congressman who was considering a Senate bid prior to Specter’s party switch, has raised questions about Specter’s motives in joining the Democratic Party, sounding a discordant note even as other leading Democrats, ranging from Obama to Pennsylvania Gov. Ed Rendell, have embraced Specter’s conversion.

The first Democratic Senate candidate to announce this year, former Constitution Center CEO Joseph Torsella, has also said he’s remaining in the Senate race — and he went so far as to launch his campaign’s website over the weekend. Despite his close ties to Rendell, Torsella has resisted calls so far to clear the field for Specter.

Union leaders are among those who haven’t yet fallen in line with Specter. Sestak met with Service Employees International Union President Andy Stern on Monday, fueling speculation that Sestak is seeking labor’s support as he considers a potential candidacy. Stern, whose group hasn’t committed to endorsing Specter in the 2010 primary, praised Sestak’s CNN appearance as “impressive” on his Twitter feed Sunday.

Richard Trumka, the secretary-treasurer of the AFL-CIO — and a third-generation Pennsylvania coal miner — said Monday that labor might not back Specter if he votes against them on EFCA, as Specter has promised to do.

“Those decisions will be made by people in the state, and our members in the state know who will stand with them,” Trumka said on the ABC News daily webcast “Top Line.” “And if Arlen Specter — he stood with them in the past — if he continues to stand with them, they’ll support him. If he doesn’t, they won’t support him.”

“There will be opposition to Specter in the Democratic ranks,” said Pennsylvania GOP political analyst Charlie Gerow. “It was believed that the governor would be able to muscle out the Democratic opposition, and that hasn’t taken place.”

Toomey hasn’t exactly caught the fancy of his party, either. Almost as soon the reality sunk in that the conservative former three-term congressman could be the GOP nominee in Democratic-trending Pennsylvania, some Republicans began casting about for an alternative.

“I don’t think there is anybody in the world who believes he can get elected senator there,” Utah Sen. Orrin Hatch, the vice chairman of the National Republican Senatorial Committee, said last week. POLITICO reported Thursday that the National Republican Senatorial Committee was in contact with moderate Pennsylvania Rep. Jim Gerlach. On Saturday, Roll Call reported that national and state Republicans were urging another moderate, former GOP Gov. Tom Ridge, to join the race.

“The irony, of course, is that Republicans are looking for that moderate candidate who can beat Toomey and hold the seat, which, if that is the case, makes Arlen Specter’s argument,” said Pennsylvania Democratic consultant Larry Ceisler. “They bought the message but not the messenger.”

In the first public poll taken since Specter’s party switch, the five-term incumbent leads Toomey in a prospective matchup by 20 points, 53 percent to 33 percent.

The Quinnipiac University survey shows Ridge would be a formidable opponent against Specter and shows Specter leading the former governor by only 3 points, 46 percent to 43 percent.

NRSC Chairman John Cornyn of Texas hesitated when given the opportunity to endorse Toomey on Friday.

“I don’t think it’s wise for me to tell Pennsylvania Republicans who their nominee should be, so I’m not going to do that,” said Cornyn. “But we want to make sure the strongest nominee is chosen so we can run them against whoever wins the Democratic nomination. Our goal is to hold onto that seat, and we’ll do whatever we can to make it happen.”